The invention relates to a frequency synthesizer in the 36 to 38.6 GHz millimeter wave frequency range, and in particular to an automatic loop gain correction system and a sophisticated lock search system, both of which belong to the frequency translation loop.
Indirect synthesizers in this range suffer from the instability of the frequency translation loop parameters because of the gain (.DELTA.F/.DELTA.V) variations of the voltage controlled oscillators (VCO) at millimeter wave frequencies. This gain is not constant; it varies with temperature and time and it is not identical in different oscillators, resulting in gain variations of tens of dB. On the other hand, because of the narrow frequency tuning range and the high cost of millimeter wave oscillators, it is desirable to use one oscillator and to exploit its entire frequency tuning range, in which the gain variation is also several tens of dB. The oscillator gain variations cause variations in the loop gain, resulting in locking with undesirable loop parameters or failure to lock at all. In the past, various complicated methods were developed to overcome this problem, making wide use of low-frequency synthesizers together with multipliers and/or harmonic mixers, or the unavoidable use of several oscillators each operating in its linear frequency tuning range which is only part of the entire range, or the use of an open loop linearizer adapted specially to any VCO with the attending limited operation caused by the severe variations of the oscillator gain.
The subject of this invention is an indirect synthesizer, which, due to the automatic loop gain correction and sophisticated lock search systems, enables to overcome the above problems. The automatic loop gain correction system makes it possible to ignore variations of VCO's parameters or parameters of any other component in the loop (due to either differences between oscillators, temperature variations, frequency tuning or any other effect). As a result, the entire frequency tuning range of the oscillator can be utilized, while control of the loop gain makes possible a constant frequency translation loop bandwidth, and thus optimum noise and spurious frequencies products. The sophisticated lock search system allows coverage of the entire oscillator control voltage variation range without causing severe phase stress and without dependence on the dc gain between the phase detector output and the oscillator control voltage input. This gain varies, in order to compensate the change in the millimeter wave oscillator gain, because of the need to maintain constant loop gain. After the initial loop lock, the loop parameters are measured continuously; if changes occur in these parameters, the loop gain is corrected as is the locking voltage supplied by the search system, without loss of lock and without any disturbance in the loop.
Because the oscillator operates at a high frequency, it has a relatively very high sensitivity to frequency tuning due to changes in the control voltage. Thus, the control voltage must be highly free of noise, including noise generated by the analog components of the loop. Such performance is attained by correctly locating the blocks in the loop, selecting and designing correctly the closed loop components.
All the above circuits are implemented simply, using inexpensive components with low current drains, which are so simple that no microcomputer, software, etc., is required.